Quantum calculations of unusual mode specificity in H+C2H2→H2+C2H

Abstract

We report reduced dimensionality coupled channel scattering calculations of rotationally averaged cross sections for the H+C2H2↔H2+C2H reaction. A new ten degree-of-freedom potential is developed for use in these calculations. This potential surface is based on a previous potential for C2H2 and C2H as well as previous ab initio calculations of the saddle point properties. We focus on the effect of exciting the symmetric and antisymmetric CH stretches, and symmetric CC stretch in C2H2 on the reactivity, and also on the vibrational distribution of the H2 and C2H products. A significant and surprising finding is that excitation of the CC stretch in combination with excitation of either the symmetric and/or antisymmetric stretch increases the reactivity significantly. A simple Franck–Condon model is used to rationalize these mode specific effects. The thermal rate constant for the H2+C2H→H+C2H2 reaction is also calculated in the temperature range from 200 to 400 K, and compared to limited experimental data.